Competition between and π- and σ-based interactions in metal ion complexes of the phenyl radical

Abstract

Hybrid density functional theory (B3-LYP and B3-PW91) and counterpoise-corrected ab initio (MP2) calculations have been performed for the adducts of the main-group metal ions Na+, Mg+, Al+, K+, and Ca+ with the phenyl radical C6H5. While the calculated bond dissociation energies (BDEs) and complexation geometries of the alkali metal ion adducts do not differ substantially from those determined for the analogous M+/benzene π-complexes, the alkaline earth ions exhibit a strong preference for σ-bond formation at the bare C atom of the phenyl ring, with calculated BDEs exceeding those determined for benzene complexation by approximately 80 and 150kJmol-1 for Mg+ and Ca+, respectively. This σ-coordination is only feasible when the unpaired spins on the reactant Mg+ (or Ca+) and C6H5 radicals are opposed, thereby leading to a closed-shell (singlet state) adduct ion. When the unpaired spins are aligned, the triplet-state adduct thus produced adopts π-complex minimum energy geometry with a BDE significantly below that of the corresponding M+/benzene complex. Both π- and σ-coordination are also found to be local minima on the AlC6H5+ potential energy surface, with the BDE of the Al+/C6H5 σ-complex being approximately 50kJmol-1 larger than that of the corresponding π-complex, and about 20kJmol-1 above the BDE for Al+/benzene. The overall results of our study show that the phenyl radical exhibits a much greater degree of selectivity between the main-group metal ions represented here than is evident for benzene. The phenyl radical BDEs range from 59.7 (K+) to 264.7kJmol-1 (Ca+) according to calculations at the B3-PW91/6-311+G(2df,p) level of theory, which contrasts with the 65.5 (K+) to 147.9kJmol-1 (Al+) BDE values determined for benzene at the same level of theory. Show more